Relay lens systems made of a plurality of rod lenses arranged in succession are used in many rigid endoscopes. Each element of the relay lens system, in particular each rod lens, brings about a complete reversal of the image, i.e. a lateral reversal in both vertical and horizontal direction, equating to a rotation through 180 degrees about the optical axis of the relay lens system. In the case of direct-vision endoscopes or endoscopes with a viewing direction parallel to the longitudinal axis of the shaft, an erect and non-reversed image becomes visible in the eyepiece for the user due to a suitable (in particular odd) number of elements of the relay lens system or rod lenses. This also applies to non-direct-vision endoscopes, which have a Bauemfeind prism or a different arrangement of an even number of coplanar reflective surfaces for deflecting the light emanating from an object.
Endoscopes with an adjustable viewing direction for example only have one reflective surface at the distal end, which reflective surface is pivotable for adjusting the viewing direction (cf., for example, DE 10 2010 010 948 A1). The single reflective surface brings about a lateral reversal in a direction parallel to a plane which contains the surface normal of the reflective surface and the optical axis in front of and behind the reflective surface.
Alternatively, provision is made, for example, for two reflective surfaces at the distal end of the endoscope, of which one is pivotable for adjusting the viewing direction (cf., for example, DE 10 2010 033 425 A1). When pivoting the pivotable reflective surface and hence the viewing direction, the observed image rotates.
For the image erecting or for counter rotating the image, use can be made of a Dove/Amici prism or an Abbe/König prism. Both prisms have specific disadvantages.